1. Field of the Invention
The present invention relates to an optical recording medium that has a recording layer made of an organic compound and with which recording and playback is possible using a laser beam having a specific wavelength.
2. Description of the Related Art
A CD-R is a writable, optical information recording medium (optical disc) on which information can be written only once by irradiation with laser light, and is widely known. This medium is advantageous in that information recorded on the CD-R can be played back using a commercial CD player, and the demand for CD-Rs has been escalating recently with the spread of personal computers. Further, writable digital versatile discs (DVD-Rs), on which digital high-vision images can be recorded, have also been put to practical use as media having a larger recording capacity than CD-Rs.
Such writable optical recording media typically comprise a disc substrate having successively disposed thereon a light-reflective layer (formed of, for example, Au), a recording layer formed of an organic compound, and a resin layer (may be referred to as a “cover layer” below) including an adhesive for adherence to the recording layer. Information is recorded on the disc by irradiating the disc with a laser from the side of the disc at which the resin layer is disposed. Specifically, the portion of the recording layer that is irradiated absorbs light, whereby temperature increases at the irradiated portion. The local increase in temperature deforms the recording layer (e.g., formation of pits) at the irradiated portion, whereby information is recorded. The recorded information is ordinarily reproduced by irradiating the disc with a laser having the same wavelength as that of the laser used to record the information, and by detecting a difference in reflectance between the region of the recording layer that has been deformed (recorded portion) and the region of the recording layer that has not been deformed (unrecorded portion).
Recently, high-vision television and networks such as the Internet have rapidly become more wide spread. In addition, the start of HDVT (High Definition Television) broadcasting is near at hand. As a result, large-capacity optical recording media capable of recording visual information easily and inexpensively are in demand. While DVD-Rs currently play a significant role as large-capacity recording media, the demand for media having greater recording capacity and higher density continues to escalate, and development of recording media that can cope with this demand is also needed. For this reason, development of recording media having ever greater storage capacity with which high-density recording can be effected with short wave light continues to advance. In particular, writable optical recording media on which information can be recorded only once are frequently used to backup or store large volumes of information over a long period of time. Therefore, the demand for such media development is high.
Ordinarily, the density of optical recording media can be increased by shortening the wavelength of the laser used for recording and playback, and by reducing beam spots through increasing NA (numerical aperture) of objective lenses. Recently, from red semiconductor lasers having a wavelength of 680 nm, 650 nm or 635 nm, to bluish purple semiconductor lasers (may be referred below as “bluish purple laser”) that have a wavelength of 400 nm to 500 nm and are capable of super high-density recording, development has rapidly advanced. The development of optical recording media corresponding to these lasers is also being conducted. In particular, the development of optical recording systems using bluish purple lasers and high NA pickups has been explored since bluish purple lasers hit the market, and rewritable optical recording media and optical recording systems having phase change recording layers have already been announced as a DVR system (“ISOM2000”, pp. 210-211). Thus, some progress has been made with respect to increasing the density of rewritable optical recording media.
In optical recording media for optical recording systems using bluish purple lasers and high NA pickups, it is preferable to reduce the thickness of the cover layer on which the laser is made incident in order to focus the high NA objective lens when the recording layer is irradiated with the bluish purple laser. For example, a thin film is used as the cover layer and adhered to the recording layer with an adhesive. The combined thickness of the cover layer and an adhesive layer formed by curing the adhesive is approximately 100 μm, and optimized in accordance with the wavelength of the laser used and NA.
However, in write-once optical recording media recorded and played back by optical recording systems using bluish purple lasers and high NA pickups, the use of an organic compound in the recording layer is problematic in that satisfactory recording characteristics are unobtainable.
While the cause of this problem is not entirely clear, it is assumed that fine unevenness occurs in the recording layer because the organic compound in the recording layer becomes dissolved into the adhesive of the adhesive layer.